青藏高原南部拉薩地體的變質(zhì)作用與動力學(xué)

董 昕, 張澤明, 向 華, 賀振宇

中國地質(zhì)科學(xué)院地質(zhì)研究所, 北京 100037

青藏高原是特提斯洋盆多期擴(kuò)張與消減和其間陸塊多期俯沖-碰撞作用的產(chǎn)物, 具有長期、復(fù)雜的地質(zhì)演化歷史(圖1)。一般認(rèn)為, 青藏高原自北向南由松潘—甘孜、羌塘、拉薩地體和喜馬拉雅帶組成,它們之間依次為金沙江(JS)、班公湖—怒江(BN)和印度—雅魯藏布江(IYZ)縫合帶, 這些縫合帶分別代表古特提斯、中特提斯和新特提斯洋盆的殘余(圖1,2)。研究表明, 拉薩地體是由南、北拉薩地體拼合而成的, 它們之間為洛巴堆—米拉山斷裂(LM), 羌塘地體是由南、北羌塘地體拼合而成的, 它們之間為龍木措—雙湖縫合帶(LS)(圖2)。

拉薩和羌塘地體在古生代位于岡瓦納超大陸北緣, 它們與亞洲之間為古特提斯洋(圖 1a, 3)。在中生代早期, 隨著中特提斯洋和新特提斯洋的打開,羌塘和拉薩地體從岡瓦納大陸北緣分離, 向北漂移,并在侏羅紀(jì)時期匯聚到歐亞板塊南緣(圖1b, c)。在早白堊紀(jì), 南印度洋打開, 印度板塊從澳大利亞和南極板塊分離(圖 1d)。從白堊紀(jì)開始, 隨著新特提斯洋向歐亞大陸之下俯沖, 印度板塊迅速向北漂移,并在約55 Ma與位于歐亞板塊南緣的拉薩地體發(fā)生碰撞, 形成了青藏高原和喜馬拉雅造山帶。因此, 拉薩地體是揭示青藏高原形成與演化歷史的關(guān)鍵之一。但是, 以往的研究普遍認(rèn)為拉薩地體中的變質(zhì)巖形成于前寒武紀(jì), 并沒有意識到它們是拉薩地體顯生宙多期造山作用的產(chǎn)物, 忽視了它們在揭示拉薩地體形成與演化歷史中的重要限定作用。

我們最近幾年研究發(fā)現(xiàn), 拉薩地體經(jīng)歷了多期和不同類型的變質(zhì)作用, 從北向南, 拉薩地體包括四個變質(zhì)帶(圖2), 它們分別是北部的安多高壓變質(zhì)帶, 形成在早侏羅紀(jì)陸-陸碰撞造山過程中, 中部的納木措高壓變質(zhì)帶, 形成在晚新元古代洋殼俯沖過程中, 南、北拉薩地體之間的松多高壓/中壓復(fù)合變質(zhì)帶, 形成在晚古生代的洋殼俯沖至早中生代陸-陸碰撞造山帶過程中, 南部的岡底斯高溫/中壓復(fù)合變質(zhì)帶, 形成在中、新生代的洋殼俯沖到陸-陸碰撞造過程中(Zhang et al., 2013)。這些變質(zhì)作用及其同時發(fā)生的巖漿作用表明拉薩地體經(jīng)歷了從新元古代到新生代的長期構(gòu)造演化。

1 北拉薩地體中部的新元古代的高壓變質(zhì)作用, 拉薩地體起源與早期構(gòu)造演化

圖1 拉薩地體與特提斯洋形成演化的古地理圖(據(jù)Gehrels et al., 2011)Fig.1 Paleogeographic maps of the evolution of the Lhasa terrane and Tethyan Ocean (after Gehrels et al., 2011)

新元古代的高壓變質(zhì)帶分布于北拉薩地體中部,與古生代的變質(zhì)沉積巖之間呈斷層接觸關(guān)系。高壓變質(zhì)巖主要由基性麻粒巖組成, 含少量片麻巖、片巖和大理巖(張澤明等, 2010)。巖石學(xué)和年代學(xué)研究表明, 高壓基性麻粒巖的原巖為形成于新元古代(～900 Ma)的輝長巖, 并具有典型洋中脊型玄武巖的地球化學(xué)特征。這些巖石經(jīng)歷了新元古代晚期(～650 Ma)的高壓變質(zhì)作用, 并疊加了早古生代的中壓型變質(zhì)作用(Dong et al., 2011a; Zhang et al.,2012a)。相關(guān)的證據(jù)很可能說明北拉薩地體部分地起源于東、西岡瓦納超大陸之間的莫桑比克洋的北部, 在超大陸的匯聚過程中發(fā)生了深俯沖和高壓變質(zhì)作用。因此, 位于印度大陸西北緣的北拉薩地體很可能是東非造山帶的一部分, 分布在北拉薩地體、馬達(dá)加斯加、南印度、東非和東南極的高壓變質(zhì)巖構(gòu)成了一條巨型的南北向高壓變質(zhì)帶(圖 3;Zhang et al., 2012a, 2013)。

2 南、北拉薩地體之間的三疊紀(jì)中壓變質(zhì)帶與早中生代碰撞造山作用

在南、北拉薩地體之間分布的一套中、高級變質(zhì)巖以前被認(rèn)為是拉薩地體的前寒武紀(jì)結(jié)晶基底,被命名為念青唐古拉群。實際上, 這套巖石的原巖主要由古生代的沉積巖組成, 它們經(jīng)歷了三疊紀(jì)的中壓角閃巖相變質(zhì)作用, 并伴隨有同變質(zhì)的巖漿作用和強烈的變形作用。結(jié)合二疊紀(jì)榴輝巖在這個變質(zhì)帶中的產(chǎn)出, 拉薩地體中部存在一條東西向的板塊匯聚邊界(圖2)。古特提斯洋的俯沖和南、北拉薩地體的拼合導(dǎo)致了晚古生代到中生代的造山 作用(Yang et al., 2009; Zeng et al., 2009; Zhu et al., 2011;Dong et al., 2011b; Zhang et al., 2013)。

圖2 拉薩地體變質(zhì)簡圖Fig.2 Metamorphic map of the Lhasa terrane

圖3 岡瓦納超大陸及東非造山帶、拉薩和羌塘地體再造圖(據(jù)Zhang et al., 2013)Fig.3 Reconstruction of the East African orogen, Lhasa and Qiangtang terranes (after Zhang et al., 2013)

3 晚白堊紀(jì)的紫蘇花崗巖與新特提斯洋中脊俯沖造山作用

晚白堊紀(jì)的紫蘇花崗巖分布于岡底斯巖漿弧的東南部, 以前被認(rèn)為是中新元古代的麻粒巖。紫蘇花崗巖由中長石、微斜長石、頑火輝石、透輝石、石英、黑云母和角閃石組成, 為典型的鎂質(zhì)、鈣堿性、準(zhǔn)鋁質(zhì)紫蘇花崗巖, 并具有埃達(dá)克巖的地球化學(xué)特征。鋯石U-Pb 定年給出的結(jié)晶年齡為85 Ma。紫蘇花崗巖的礦物中含高密度的CO2流體包裹體和赤鐵礦包體。相關(guān)證據(jù)表明, 紫蘇花崗巖形成在干的、高氧逸度、高溫(850 ～ 950°C)和高壓(0.7 ～1.0 GPa)條件下, 很可能形成在新特提斯洋中脊俯沖形成的板片窗構(gòu)造環(huán)境。軟流圈沿板片窗上涌為紫蘇花崗巖和同期麻粒巖相變質(zhì)作用的形成提供了有力條件(張澤明等, 2009; Zhang et al., 2010a, b,2011)。

4 晚泥盆紀(jì)的花崗巖與岡瓦納超大陸北緣的古生代造山作用

晚泥盆紀(jì)至石炭紀(jì)花崗巖分布在拉薩地體南部,很可能是岡瓦納超大陸北緣古生代安第斯型造山作用的產(chǎn)物(圖3; 董昕等, 2010)。大多數(shù)研究認(rèn)為, 從泥盆紀(jì)至石炭紀(jì), 岡瓦納大陸北緣處于一個相對寧靜的構(gòu)造期(Gehrels et al., 2011), 但在拉薩和羌塘地體中陸續(xù)發(fā)現(xiàn)的中、晚古生代巖漿作用, 為岡瓦納大陸北緣的構(gòu)造演化提供了新信息(董昕等, 2010;Pullen et al., 2011; Ji et al., 2012)。

5 岡底斯巖漿弧深部的變質(zhì)作用與增生造山

在拉薩地體南部廣泛分布的中、新生代岡底斯巖漿弧形成在新特提斯洋殼向北俯沖的安第斯型造山過程中, 為大陸地殼顯著生長的產(chǎn)物。研究發(fā)現(xiàn),與岡底斯巖基伴生的高級變質(zhì)巖同樣形成在中、新生代, 與中、基性深層侵入巖一起構(gòu)成了岡底斯巖漿弧的中、下地殼組成。這表明幔源巖漿巖的不斷增生導(dǎo)致了巖漿弧地殼的加厚和加熱, 形成了同侵入期的高壓高溫麻粒巖相變質(zhì)作用、深熔作用和 S型花崗巖, 顯示出增生造山帶地殼生長與巖漿作用、變質(zhì)作用之間的成因聯(lián)系(薛光琦等, 2005; 王金麗等, 2008, 2009; 董昕等, 2009, 2012; 李奮其等,2010; 唐菊興等, 2012; Dong et al., 2010; Zhang et al.,2010a, b, 2012b; Guo et al., 2011, 2012; 劉峰等,2011; Searle et al., 2011; Guan et al., 2012)。

6 安多地塊的多期構(gòu)造熱事件與造山作用記錄

位于拉薩地體北部的安多地塊普遍經(jīng)歷了高級變質(zhì)作用, 被認(rèn)為是拉薩地體的結(jié)晶基底。研究表明, 安多地塊最老巖石為新元古代(820 Ma)的巖漿巖, 形成在環(huán) Rodinia超大陸周緣的安第斯型造山過程中(Guynn et al., 2012; Zhang et al., 2012c)。安多地塊發(fā)育早古生代(～500 Ma)的雙峰式巖漿巖, 很可能形成在環(huán)岡瓦納超大陸周緣的安第斯型造山過程中(圖3; Zhang et al., 2012c)。安多地塊在侏羅紀(jì)經(jīng)歷了深俯沖和高壓麻粒巖相變質(zhì)作用, 很可能發(fā)生在中特提斯大洋巖石圈閉合過程中(Zhang et al.,2012c, 2013)。

上述研究成果表明, 拉薩地體中北部含有新元古代的洋殼殘余, 并經(jīng)歷了新元古代至早古生代的高壓/中壓變質(zhì)作用, 拉薩地體中部經(jīng)歷了晚古生代至早中生代的高壓/中壓型變質(zhì)作用, 拉薩地體南部經(jīng)歷了晚中生代至新生代的多期高溫變質(zhì)作用, 拉薩地體北部的安多地塊經(jīng)歷了早中生代的高壓變質(zhì)作用。這些變質(zhì)作用記錄揭示出, 拉薩地體經(jīng)歷了多期洋-陸或陸-陸匯聚造山作用過程, 豐富了青藏高原前新生代的構(gòu)造演化歷史, 成為研究匯聚板塊邊緣變質(zhì)作用與動力學(xué)典型實例。

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TANG Ju-xing, Dorji, LIU Hong-fei, LANG Xing-hai, ZHANG Jin-shu, ZHENG Wen-bao, YING Li-juan.2012.Minerogenetic Series of Ore Deposits in the East Part of the Gangdise Metallogenic Belt[J].Acta Geoscientica Sinica, 33(4):393-410(in Chinese with English abstract).

WANG Jin-li, ZHNAG Ze-ming, SHI Cao.2008.Anatexis and dynamics of the soutehern Lhasa terrane[J].Acta Petrologica Sinica,24(7): 1539-1551(in Chinese with English abstract).

WANG Jin-li, ZHANG Ze-ming, DONG Xin, LIU Feng, WANG Wei, XU Fang-jian, SHEN Kun.2009.Discovery of late Cretaceous garnet two-pyroxene granulite in the southern Lhasa terrane, Tibet and its tectonic significances[J].Acta Petrologica Sinica, 25(7): 1695-1706(in Chinese with English abstract).

XUE Guang-qi, WANG You-xue, SU He-ping.2005.Interpretation of the Low Velocity Anomalies in the Lhasa Terrain: Tomorgraphic Evidence for Partial Melting in the Crust[J].Acta Geoscientica Sinica, 26(5): 411-415(in Chinese with English abstract).

YANG Jing-sui, XU Zhi-qin, LI Zhao-li, XU Xiang-zhen, LI Tian-fu, REN Yu-feng, LI Hua-qi, CHEN Song-yong,ROBINSON P T.2009.Discovery of an eclogite belt in the Lhasa block, Tibet: A new border for Paleo-Tethys?[J].Journal of Asian Earth Sciences, 34(1): 76-89.

ZHANG Ze-ming, WNAG Jin-li, DONG Xin, ZHAO Guo-chun,YU Fei, WANG Wei, LIU Feng, GENG Guan-sheng.2009.Petrology and geochronology of the charnockite from the southern Gangdese belt, Tibet: Evidence for the Andean-type orogen[J].Acta Petrologica Sinica, 25(7): 1707-1720(in Chinese with English abstract).

ZHANG Ze-ming, DONG Xin, GENG Guan-sheng, WANG Wei,YU Fei, LIU Feng.2010.Precambrian metamorphism of the northern Lhasa terrane, south Tibet and its tectonic implications[J].Acta Geologica Sinica, 84(4): 449-456(in Chinese with English abstract).

ZHANG Ze-ming, ZHAO Guo-chun, SANTOSH M, WANG Jin-li,DONG Xin, SHEN Kun.2010a.Late Cretaceous charnockite with adakitic affinities from the Gangdese batholith, southeastern Tibet: Evidence for Neo-Tethyan mid-oceanic ridge subduction?[J].Gondwana Research, 17(4): 615-631.

ZAHNG Ze-ming, ZHAO Guo-chun, SANTOSH M, WANG Jin-li,DONG Xin, LIOU J G.2010b.Two-stages of granulite-facies metamorphism in the eastern Himalayan syntaxis, south Tibet:Petrology, zircon geochronology and implications for the subduction of Neo-Tethys and the Indian continent beneath Asia[J].Journal of Metamorphic Geology, 28(7): 719-733.

ZHANG Ze-ming, SHEN Kun, SANTOSH M, DONG Xin.2011.High density carbonic fluids in a slab window: Evidence from the Gangdese charnockite, Lhasa terrane, southern Tibet[J].Journal of Asian Earth Sciences, 42(3): 515-524.

ZHANG Ze-ming, DONG Xin, LIU Feng, LIN Yan-hao, YAN Rong,HE Zhen-yu, SANTOSH M.2012a.The making of Gondwana:Discovery of 650 Ma HP granulites from the North Lhasa, Tibet[J].Precambrian Research, 212-213: 107-116.

ZHANG Ze-ming, DONG Xin, SANTOSH M, LIU Feng, WANG Wei, YU Fei, HE Zhen-yu, SHEN Kun.2012b.Petrology and geochronology of the Namche Barwa Complex in the eastern Himalayan syntaxis, Tibet: Constraints on the origin and evolution of the north-eastern margin of the Indian Craton[J].Gondwana Research, 21(1): 123-137.

ZHANG Ze-ming, DONG Xin, LIU Feng, LIN Yan-hao, YAN Rong,SANTOSH M.2012c.Tectonic Evolution of the Amdo Terrane,Central Tibet: Petrochemistry and Zircon U-Pb Geochronology[J].The Journal of Geology, 120(4): 431-451.

ZHANG Ze-ming, DONG Xin, SANTOSH M, ZHAO Guo-chun.2013.Metamorphism and tectonic evolution of the Lhasa terrane, Central Tibet[J].Gondwana Research, http://dx.doi.org/10.1016/j.gr.2012.08.024.

ZENG Ling-sen, LIU Jing, GAO Li-e, CHEN Fang-yuan, XIE Ke-jia.2009.Early Mesozoic high-pressure metamorphism within the Lhasa block, Tibet and implications for regional tectonics[J].Earth Science Frontiers, 16(2): 140-151.

ZHU Di-cheng, ZHAO Zhi-dan, NIU Yao-ling, DILEK Y, MO Xuan-xue.2011.Lhasa terrane in southern Tibet came from Australia[J].Geology, 39(8): 727-730.